1. Field of the Disclosure
The invention concerns calibration methods for imaging systems and more especially methods for calibration in the field of image capture systems. It further concerns imaging systems implementing such calibration methods.
2. Background of the Invention
Variable focus image capture systems are becoming increasingly integrated in picture acquisition devices and data capture devices. Such devices include but are not limited to 1D and 2D barcode readers, machine vision image capture devices, fingerprint or iris recognition systems. Focusing ability is becoming a must-have feature for those image or data capture devices. For instance, 2D barcodes are becoming increasingly common, and their decoding requires imaging devices instead of laser scanners.
Such a variable focus image capture system usually comprises a camera module composed of a CMOS or CCD sensor (matrix or linear), an imaging lens module, a focusing actuator, and a distance measurement device.
When the user of the system wants to take a picture of an object, the distance measurement device determines the distance from the system to the object and sends the right command to the actuator so that the optical module can focus onto the object and therefore maximize image quality. Also, using this distance measurement device maximizes aggressiveness, i.e., the time to capture the data and to process it, in other words the time to decode the image information. Typical distances range from a few centimeters to infinity and aggressiveness is typically less than ˜0.2-0.4 sec.
FIG. 1A illustrates a schematic view of a 2D bar code reader 100. It comprises a camera module (CM) 101 characterised by an optical axis (Δ) and a given field of view (θFOV). The camera module 101 comprises a sensor 102 and a lens arrangement 103 settled in a housing 104. The lens arrangement 103, schematically represented on FIG. 1A, comprises an electrically controlled optical device (not represented on FIG. 1A), for example a liquid lens, for adjusting the focus of the device. The focus is adjusted as a function of the measured distance of the barcode 105. The imaging system further comprises a driver 106 for applying a predetermined electrical signal to the electrically controlled optical device which is a function of the measured distance, It further comprises processing means 107, e.g. an Imaging Signal Processor, usually called “ISP”, that will process the image and control the sensor parameters. A user interface 108 is connected to the processing unit 107 via a decoder 109. A nearly collimated laser beam 110, emitted by a laser source 111, and making a given angle with the optical axis of the imaging system, crosses the field of view of the imaging system in such a way that the image of the laser spot reflected from a barcode located at far and near distances (B and A points respectively) will move over the field of view of the imaging device (B′ and A′ points in the sensor plane). The image of the laser spot will thus be translated over the sensor height or width. In measuring the position of the centroid of the laser spot on the sensor, the distance can be computed based on a preliminary calibration. A memory 112 stores the parameters of the calibration. A power supply 113 provides the electrical power to the different elements of the system.
In such applications, focusing speed, or time to focus, is a critical parameter. Conventional auto-focus methods, where the actuator command is dynamically optimized depending on sensor feedback, cannot be used because such closed-loop driving requires several steps (images acquisition) to achieve focusing through the commonly called full scan search, leading to a very long time to focus not suited to cited applications.
Thus, the use of an external device is required to determine directly the right command to send to the actuator. In the case of a focusing lens module, this external device is the distance measurement device.
When the optical module is used in addition to a distance measurement device, it is possible to adjust the module to focus on the object that is at the measured distance. This also refers to what is commonly called “open-loop” systems, wherein no feedback about an output is taken into account to generate the output, and wherein an external input data can be used, for example a distance measurement, to generate the output. It leads to extremely fast time to focus as only one command on the actuator is required. In comparison, a close-loop system based on an autofocus loop requires several steps, including the acquisition of images at a fixed frame rate of typically 15 to 60 Hz, and takes much more time—typically 0.5 to 1 second.
On the other hand, open-loop driving of the module requires storing the distance/actuator command relationship in the actuator's driving system, which is usually stored in the memory chip 112 or in a computer.
Let us take the example of a liquid lens, which is a voltage-driven focusing actuator. Such liquid lens is described for example in European Patent Application EP 1662276 in the name of the applicant. It comprises a refractive interface between first and second immiscible liquids that is movable by electrowetting. More precisely, as detailed in the above mentioned reference, a liquid lens often comprises two transparent windows, wherein said windows can be fixed lenses in some embodiments, arranged in parallel and facing each other, and delimiting, in part, an internal volume containing two immiscible liquids with different optical indices. Where the two liquids meet they form an optical interface in the form of a meniscus, which can have a number of different shapes. The liquids have substantially equal densities, and one is preferably an insulating liquid, for example comprising oil and/or an oily substance, and the other is preferably a conductive liquid comprising for example an aqueous solution.
The distance/actuator command relationship for such a liquid lens is shown in FIG. 1B. For a liquid lens, the command is voltage; for mechanical actuators it can be current. This context can be applied to any kind of command. As it is shown on FIG. 1B, the relationship is linear, or quasi linear, for a focusing distance from Distance 1 to Distance 2. The voltage values to reach Distance 1 and Distance 2 depend on every single actuator. As the response is linear, only two values are needed in this particular case to completely determine the relationship
The distance/actuator command relationship is stored as a look-up table, in every unit that includes the variable focus module, at the end of production, thanks to a calibration process. But, during the lifetime of the unit, this initial calibration may not be relevant anymore. Indeed, if the characteristics of the unit change over time, the look-up table may have to take these changes into account so that the best performance is guaranteed over time. Further, the look-up table may vary over the working temperature range of the device (e.g. −20 to +60° C. for industrial devices). In such cases a temperature sensor is embedded in the system and its output is used to adjust the values of the look-up table.
A problem to solve is the calibration of each individual unit at the end of production, as well as periodical recalibration during the lifetime of the product. Since it is too costly to send the unit back to the manufacturer for recalibration, a calibration system that is included in the device is a big advantage.